The present invention relates to a spherical carbon material, a spherical activated carbon and processes for preparing the two, respectively.
Hitherto, for preparing spherical carbon materials or spherical activated carbon, a method of molding a mixture of powdery carbon and a binding agent or a method in which a viscosity-controlling agent is mixed into petroleum pitch or coal-tar pitch and, after melt-molding the mixture in spherical shape, the viscosity-controlling agent is removed from the shaped material by extraction with a solvent, and in which the spherical pitch is then infusibilized, baked and/or activated has been practicized (Japanese Patent Publication No. 18879/75). Particularly, according to the last-mentioned method, a spherical carbon material small in apparent density, hardly pulverizable into minute powder and applicable as a carbon material for various fields is obtained. More specifically, the spherical activated carbon obtained by the last-mentioned method is effectively applicable for treating waste water and exhaust gases.
However, the last-mentioned method cannot supply a spherical carbon material or spherical activated carbon excellent in crushing strength and abrasion strength.
In addition, the productive efficiency of the last-mentioned method is reduced to a large extent due to the long time period for infusibilization in the case of using a pitch of low softening point, and it is extremely difficult to prepare a spherical carbon material or spherical activated carbon of high mechanical strength from low-quality pitch according to the last-mentioned method. Furthermore, since the melted mixture of the starting material, that is, pitch and the viscosity-controlling agent, is formed into spherical particles by shearing force in the aqueous medium, the distribution of their size represented by the diameter of the particle is broad. Accordingly, in the case where the spherical product with a uniform size is needed, it is necessary to provide an additional step of sifting.
For instance, in the case of treating waste water in a fixed-bed system or fluidized bed composed of such spherical activated carbon, crushing of the spherical activated carbon frequently occurs at the low part of the above-mentioned fluidized bed resulting in the clogging of the bed or the passage due to the thus powdered activated carbon, in the case where the crushing strength and abrasion strength of the spherical activated carbon are not sufficient. On the other hand, with a spherical activated carbon high in crushing strength and abrasion strength, an extremely effective treatment of waste water, etc. is possible. In addition, the field of application of spherical carbon material or spherical activated carbon having high mechanical strength is very wide, and such shaped carbon material and activated carbon have been strongly desired.
It is an object of the present invention to provide spherical carbon material (or a spherical activated carbon) prepared by the process comprising the steps of:
heating while continuously stirring an admixture of (a) 100 parts by weight of a mixture of 30 to 95% by weight of a pitch and 5 to 70% by weight of amorphous carbon particles of a diameter of less than 200 microns and (b) 5 to 50 parts by weight of a viscosity-controlling agent, thereby fluidifying uniformly said admixture;
extruding the thus fluidified admixture from an extruding die, thereby forming the fluidified admixture into a string-like shape;
cooling and cutting the thus formed string-likely shaped admixture into fragments of stick-like shape;
putting the thus obtained stick-likely shaped fragments into hot water kept at a temperature higher than the softening point of said admixture thereby obtaining spheres from said admixture; and
subjecting the thus obtained spheres to a series of treatments for removal of said viscosity-controlling agent, infusibilization by the use of a gaseous oxidant or of an aqueous solution of an oxidant and carbonization (and/or activation).
Another object of the present invention is to provide a process for preparing a spherical carbon material (or a spherical activated carbon), comprising the above-mentioned steps.
As a petroleum pitch for use in the present invention, for instance, pitches obtained by thermal cracking of petroleum products such as crude oil, heavy oil, naphtha, asphalt, light oil and kerosene, or pitches obtained by thermal polymerization of the above-mentioned petroleum products are mentioned. As a coal-tar pitch for use in the present invention, polymerized products of high pitch and medium pitch are mentioned, preferably those having a softening point of higher than 140.degree. C.
Amorphous carbon particles for use in the present invention are substantially composed of amorphous carbon such as charcoal obtained by dry distillation of ligneous material such as wood, coconut-shell and sawdust, those carbons obtained by dry distillation of coal such as green-cokes and carbon black, having less than 200 microns in mean diameter. The weight ratio of the pitch to the amorphous carbon particles is 30:70 to 95:5, and the content of amorphous carbon particles in the above-mentioned admixture is preferably 5 to 50% by weight. In a case of the content of amorphous carbon particles of less than 5% by weight of the admixture, the effect of adding the amorphous carbon particles is not observed, and on the other hand, in a case of the content of amorphous carbon particles of more than 70% by weight, the viscosity of the admixture becomes large enough to cause the difficulty of spherilization. The size of the amorphous carbon particles is less than 200 microns in average diameter, preferably less than 150 microns. If it is larger than 200 microns, it is difficult to shape the spherical product because of the unevenness of the surface of the product due to the amorphous particles.
The viscosity-controlling agent for use in the present invention is selected from the group consisting of bi- and tricyclic aromatic hydrocarbon of a favorable compatibility with the pitch and of a boiling point higher than 200.degree. C., for instance, naphthalene, methylnaphthalene, phenylnaphthalene, benzylnaphthalene, methylanthracene, phenanthrene and biphenyl, or from the group consisting of mixtures of more than one of the above-mentioned compounds. Among the above-mentioned compounds, naphthalene is particularly preferable from the view point of its large ability of controlling the viscosity of the above-mentioned pitch and its easy extractability in the step of extraction by a solvent. The relative amount of the viscosity-controlling agent is 5 to 50 parts by weight to 100 parts by weight of the mixture of pitch and amorphous carbon particles, and in practice, the amount is selected from the above-mentioned range according to the weight ratio of the pitch to the amorphous carbon particles. Since the viscosity of the above-mentioned mixture becomes larger with the increase of the amount of the amorphous carbon particles in the mixture, it is preferable to increase the amount of the viscosity-controlling agent, and on the other hand, in the case where the added amount of the amorphous carbon particles is relatively smaller, it is preferable to use the viscosity-controlling agent in a relatively smaller amount. However, in the case where the amount of the agent is less than 5 parts by weight to 100 parts by weight of the mixture of the pitch and the amorphous carbon particles, the viscosity of the above-mentioned admixture is so large that the shaping is difficult and moreover, the shaped material prepared from the admixture becomes non-porous to make the infusibilization difficult. On the other hand, in the case where the amount of the viscosity-controlling agent is larger than 50 parts by weight to 100 parts by weight of the mixture of the pitch and the amorphous carbon particles, the shaped material obtained after solvent-extraction becomes too porous with macro-pores and, accordingly, friable and poor in strength.
The above-mentioned admixture consisting of pitch, amorphous carbon particles and a viscosity-controlling agent is shaped as follows:
The admixture is heated to a temperature higher than its softening point to be processed and extruded from a die in a state of string-like material, and after cooling, the thus solidified string-likely shaped admixture is crushed into pellets or small sticks and then shaped into spherical particles by putting the sticks into hot water at a temperature higher than the softening point of the admixture. By applying the above-mentioned process of spherilization, it is possible to obtain spherical particles of the admixture even if the viscosity of the admixture is considerably high, and the dispersion of the diameter of the thus obtained spherical particles is substantially narrow in range.
As an organic solvent for use in extracting the viscosity-controlling agent from the thus obtained spherical particles, those substantially not dissolving the pitch and the amorphous carbon particles but well dissolving the viscosity-controlling agent, for instance, aliphatic hydrocarbons such as hexane, heptane, naphtha and kerosene, saturated alicyclic hydrocarbons such as cyclohexane, and aliphatic alcohols such as methanol, ethanol and propanol serve as examples. In addition, extraction of the viscosity-controlling agent by an organic solvent is carried out in one step or in multiple stages. The thus obtained spherical particles, deprived of the viscosity-controlling agent, are porous.
In the next step of infusibilization of the thus obtained porous particles, an oxidant is used. The oxidants for use in this step are roughly classified into (a) gaseous oxidant, for instance, molecular oxygen, ozone, nitrogen dioxide, sulfur trioxide, air, and mixtures thereof; and (b) aqueous solutions of oxidants, for instance, an aqueous solution of acid such as, nitric acid, sulfuric acid, persulfuric acid, peracetic acid or a mixture of said acids; and an aqueous solution of salts such as permanganates, chromates, salts of halogen oxyacid or mixture thereof.
In cases of infusibilization while using a gaseous oxidant, the above-mentioned porous particles are heated in an atmosphere including the gaseous oxidant gradually at a rate of 15.degree. to 30.degree. C./hour to about 400.degree. C.
In case of infusibilization while using an aqueous solution of an oxidant, since the amorphous carbon particles in the above-mentioned porous particles contribute to infusibilization of the pitch material in the porous particles, the process of infusibilization is extremely easily carried out by using 0.2 to 100 parts by weight, preferably 0.2 to 30 parts by weight of an aqueous solution of the oxidant at a concentration of 5 to 90% by weight, preferably 5 to 70% by weight to one part of the above-mentioned porous particles at a preferable temperature of 20.degree. to 90.degree. C. In the actual process of infusibilization, the porous particles are immersed in the aqueous solution of the oxidant, or after spraying with the solution, the porous particles are kept a while at room temperature or at a temperature of 20.degree. to 90.degree. C. In cases of using a too diluted solution of the oxidant, a large amount of the aqueous solution is necessary, and on the other hand, in cases of using a too concentrated solution of the oxidant, it is difficult to control the reaction temperature of infusibilization because of the vigorous oxidation of the pitch by the oxidant. In cases of using an amount of the oxidant solution less than 0.2 parts by weight to one part by weight of porous particles, it is difficult to obtain the uniformly infusibilized material, and on the other hand, even in case of using more than 100 parts by weight of the oxidant solution to one part by weight of the porous particles, the term of infusibilization does not shorten in proportion to the amount of the oxidant solution, such an abundant use being uneconomic.
One of the merits in using the aqueous solution of oxidant is the ease of controlling the reaction of infusibilization by utilizing the sensible and latent heat of water for absorbing the heat generated by the infusibilization, since the conductivity of the aqueous solution is better than that of the atmosphere of gaseous oxidant. Furthermore, while in the case where the starting material, that is, the admixture is prepared without adding any amorphous carbon particles, it takes a long time period for completing the infusibilization even using the aqueous solution of an oxidant at a high temperature, the infusibilization can be completed in a very short time period of 30 sec to 5 hours even at a low temperature in the case where the amorphous carbon particles are admixed. The reason for the above-mentioned phenomena has not yet been elucidated, however, it may be, in part, that the surface area of spherical particles is enlarged owing to porosity of the amorphous carbon.
Generally, the time period for completing infusibilization in aqueous phase of the porous particles of the present invention naturally depends on the concentration of the oxidant, the content of the amorphous carbon particles and the reaction temperature, and the tendency is quite the same as in all reaction between a solid material and an oxidant in aqueous phase.
After completion of infusibilization, the product is washed with water to remove the remaining oxidant in/on the product. The thus obtained product shows a raised softening point and a raised rate of carbonization as compared to those of the material before infusibilization.
Even when the infusibilized product obtained in Example 10 was momentarily heated to a temperature higher than 900.degree. C. in a reductive gaseous atmosphere of water vapor or nitrogen gas, the carbonized or the activated carbon obtained by this treatment had not shown any bubbling, crushing and mutual adhering of the particles.
The spherical carbon or the spherical activated carbon obtained by carbonizing or activating the thus infusibilized product had the same quality as those obtained by the usual method.
In conclusion, the merits of the method of infusibilization using an aqueous solution of an oxidant are, in addition to the above-mentioned, the easiness of making the process system continuously operatable with more compactly designed elements of the apparatus than those in the conventional batch system, while consuming less energy.
The thus obtained spherical porous material is carbonized in an inert atmosphere at a temperature higher than 600.degree. C. to prepare spherical carbon material, or by activating the thus obtained spherical infusibilized material with an activated agent mainly composed of water vapour, spherical activated carbon of the present invention is obtained.
The spherical carbon material or the spherical activated carbon, obtained from the admixture containing 5 to 40 weight % of the amorphous carbon based on the mixture by the present invention, shows a crushing strength increased more than 1.3 times that of the material or spherical activated carbon prepared from pitch without adding the amorphous carbon particles; it has other properties not different from those of the conventional comparative products, such as adsorbing activity. In addition, since the amorphous carbon particles for use in the present invention is available at a cheap price, the process of the present invention is more economically profitable in the case where the spherically shaped activated carbon is to be prepared. Further, according to the process of the present invention, the range of pitch which can be used for preparation of the excellent spherical activated carbon is broader than that for preparing such an excellent spherical activated carbon by the conventional method.
In the following examples, the characteristic properties of specimens is determined as follows.